The present invention is directed toward an apparatus and method for increasing the effective data rate of a transmitted signal and, more particularly, toward increasing the effective data rate of a phase modulated signal.
The use of multi-level modulation techniques (phase and amplitude) are generally well known. Increasing the level of modulation at a transmitter effectively increases its data rate, i.e., increases the number of bits per symbol that may be transmitted. For instance, xcfx80/4-DQPSK (Differential Quadrature Phase Shift Keying) modulation is a well known 2-level modulation technique transmitting two bits per symbol. Another well known modulation technique, 16QAM (Quadrature Amplitude Modulation), is a 4-level modulation technique transmitting four bits per symbol. While the higher level modulations provide higher data rates for the same channel bandwidth, there is a loss in channel sensitivity which may cause distortion under high SNR (Signal-to-Noise Ratio) conditions. Thus, higher level modulations are more reasonable at relatively good channel conditions.
Modern telecom modems utilize 256QAM modulation to transmit eight bits per symbol. Thus, noise will have a strong effect on the performance of the transmission link, or data pipeline. Accordingly, in noisy channel conditions the modem will automatically revert to lower modulation techniques that have greater noise immunity.
In wireless communication systems, fully linear modulation, such as 16QAM and higher modulation levels, is difficult to design and control. This is particularly due to the large dynamic range of RF (Radio Frequency) links, or channels, present in fully linear modulation. A system using fully linear modulation techniques requires sensitive linear receivers having accurate, dynamic AGC (Automatic Gain Control) amplifiers, transmitters with linear PAs (Power Amplifiers), and other such components.
Utilizing PSK (Phase Shift Keying) type modulation enables IF (Intermediate Frequency) limited receivers to be utilized, while providing some level of modulation depth. However, one drawback is that since PSK is phase-only modulation, the second dimension of amplitude modulation available to QAM type modulations is unavailable in PSK modulation. The modulation bits per symbol in phase-only modulation (PSK) is limited by how finely one can space the points of the differential constellation around the unit circle.
The four possible phase transitions available in xcfx80/4-DQPSK modulation provide a high level of noise immunity. Thus, there is some maneuvering room when it comes to expanding xcfx80/4-DQPSK to higher bits per symbol in good channel conditions. If xcfx80/4-DQPSK is expanded to include more constellation points, the receiver at the receiving link would need to be notified that a new differential constellation spacing is being transmitted with a higher level of bits per symbol. This would require a full coordination of the transmitting and receiving units. However, when expanding xcfx80/4-DQPSK to a higher bits per symbol rate, the issue of AM (Amplitude Modulation) should be considered. As the spacing of the xcfx80/4-DQPSK differential constellation grows, so does the chance of phase transitions close to zero level amplitude, which cannot be handled by the transmitter. This limits xcfx80/4-DQPSK expansion.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A system is disclosed for providing communication across a wireless network. The system includes a transmitter transmitting a modulated signal across a radio channel, wherein the transmitter includes first input means for receiving a primary data signal to be transmitted, second input means for receiving a secondary data signal to be transmitted, a combiner associated with the first and second input means for combining the primary and secondary data signals, and a modulator receiving the combined primary and secondary data signals from the combiner and sequentially developing a modulated signal including a string of phase values with each phase value containing information relative to both the primary and secondary data signals. The system also includes a receiver for receiving the modulated signal, wherein the receiver includes means for separating the received modulated signal into primary and secondary phase values representative of the primary and secondary data signals.
The above-described system transparently increases the effective transmitted data rate using phase modulation techniques. The existing infrastructure and hardware available in a xcfx80/4-DQPSK communication system are utilized with little change in software. The system is particularly useful in relative good channel conditions and increases the perceived usefulness of communication systems utilizing xcfx80/4-DQPSK modulation techniques.
In one form, the first input means includes a first impulse stream encoder receiving the primary data signal and generating a first impulse stream representative thereof. The second input means includes a second impulse stream encoder receiving the secondary data signal and generating a second impulse stream representative thereof.
In another form, the second impulse stream encoder receives an information signal from the first impulse stream encoder indicative of the phase transition in the primary data signal. The second impulse stream generated by the second impulse stream encoder represents a modification of the phase transition in the primary data signal.
In another form, the combiner includes a summer summing the first and second impulse streams.
In another form, the modulated signal includes a DQPSK signal.
In another form, the modulator includes an IQ modulator.
In another form, the separating means includes first determining means for determining the primary phase values from the received modulated signal, and second determining means responsive to the first determining means for determining the secondary phase values from the received modulated signal.
In yet another form, the first determining means includes a phase differentiator determining differences between successive phase values in the received modulated signal, the differences including the primary phase values, and a first converter converting the primary phase values into an approximation of the primary data signal. The second determining means includes a second converter converting the approximated primary data signal into converted phase values, and a subtractor subtracting the primary phase values from the converted phase values to generate the secondary phase values.
In still another form, the second determining means further includes a third converter converting the secondary phase values into an approximation of the secondary data signal.
A method of communication across a wireless network is also provided. The method includes the steps of transmitting a modulated signal across a radio channel, the modulated signal including a string of phase values with each phase value containing information relative to primary and secondary data signals, receiving the modulated signal, and separating the received modulated signal into primary and secondary phase values representative of the primary and secondary data signals.
In one form, the step of transmitting a modulated signal across a radio channel includes the steps of receiving the primary data signal to be transmitted, receiving the secondary data signal to be transmitted, combining the primary and secondary data signals, modulating the combined primary and secondary data signals to sequentially develop the modulated signal, and transmitting the modulated signal across the radio channel.
In another form, the step of transmitting a modulated signal across a radio channel further includes the steps of generating a first impulse stream representative of the primary data signal, and generating a second impulse stream representative of the secondary data signal. The step of combining the primary and secondary data signals includes the step of combining the first and second impulse streams.
In another form, the step of combining the first and second impulse streams includes the step of summing the first and second impulse streams together.
In another form, the modulated signal includes a DQPSK signal.
In another form, the step of separating the received modulated signal into primary and secondary phase values representative of the primary and secondary data signals includes the steps of determining the primary phase values from the received modulated signal, and determining, responsive to the primary phase value determination, the secondary phase values from the received modulated signal.
In yet another form, the step of determining the primary phase values from the received modulated signal includes the steps of determining differences between successive phase values in the received modulated signal, the differences including the primary phase values, and converting the primary phase values into an approximation of the primary data signal. The step of determining, responsive to the primary phase value determination, the secondary phase values from the received demodulated signal includes the steps of converting the approximated primary data signal into converted phase values, and subtracting the primary phase values from the converted phase values to generate the secondary phase values.
In still another form, the step of determining, responsive to the primary phase value determination, the secondary phase values from the received modulated signal further includes the step of converting the secondary phase values into an approximation of the secondary data signal.
It is an object of the present invention to increase the effective transmitted data rate in a communication system using phase modulation techniques.
It is a further object of the present invention to increase the effective transmitted data rate in a xcfx80/4-DQPSK communication system.
It is a further object of the present invention to transparently increase the effective transmitted data rate in a xcfx80/4-DQPSK communication system.
It is a further object of the present invention to increase the effective transmitted data rate in a xcfx80/4-DQPSK communication system utilizing the existing infrastructure and hardware, with little change in software.
It is still a further object of the present invention to increase the perceived usefulness of xcfx80/4-DQPSK communication systems.
It is yet a further object of the present invention to increase the effective transmitted data rate in xcfx80/4-DQPSK communication systems in relatively good channel conditions.
Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.